The present invention relates in general to wireless communication systems and in particular to a code division multiple access (CDMA) protocol named Slotted Aloha for transmitting packets of data between a mobile subscriber or station (MS) and a base station (BS) over an access channel slot comprising a preamble portion and a message capsule portion.
In prior art CDMA wireless systems, a mobile station (MS) would access a base station (BS) using a protocol named slotted Aloha.
When a MS is turned ON, it synchronizes itself to receive forward link (FL) transmissions from the BS as part of an initialization process. This synchronization is maintained as the MS moves about the cell generated by transmissions from the BS. Because this synchronization is maintained, the BS may send data to any specific MS or to all MSs in the vicinity of the BS in accordance with standardized protocols. This synchronization process is not applicable to transmissions over the reverse link (RL) from a MS to a BS.
An MS may contact or access a BS to register when entering a system foreign to its registered home system or to originate a service like a voice call and so forth. Thus CDMA wireless systems provide for allowing an MS to access the BS over an access channel during any one of consecutive time periods defined as access channel slots in accordance with the previously referred slotted Aloha protocol. If an MS is not making an outgoing call, it may remain in an ON condition all day and never use the access channel.
To perform the access function, the mobile sends an access probe comprising one time slot including a preamble time portion and a message capsule time portion. During a portion of the preamble time, the base station searches for a mobile station transmission. This portion is called the search window. If a mobile station transmission is detected by the BS, then the BS will try and synchronize to the mobile station""s transmission during the preamble time.
After the preamble portion of the time slot is completed, the base station will decode the message capsule.
The slotted Aloha protocol is known as a random access protocol which may experience collisions due to more than one mobile station attempting to access the base station during the same time slot. This condition may be alleviated by designing the hardware to accommodate more than one mobile station per slot. Such action or designing is outside the scope of this invention.
In the slotted Aloha protocol, implemented in current CDMA mobile phone systems, of the prior art, the preamble consists of a multiple of 20 millisecond frames. The message capsule also consists of a plurality of 20 millisecond frames. The number of 20 millisecond frames in each of the preamble and the message capsule portions of the time slot may be adjusted on a base station by base station basis throughout the wireless system in accordance with the radio environment for each base station. In other words, certain poor quality radio environments or environments where the base station covers a large area may require a longer preamble than base stations having a smaller area or better radio environment. Further, it may be that mobile station subscribers located in the area of some base stations typically transmit longer messages than occurs at other base stations. In such a situation, it may be more expedient to have more frames in the message capsule portion of the access channel slot for those base stations than for other base stations typically having shorter messages. The efficiency of this protocol is bounded by equation 1 where M is the number of message capsule frames and P is the number of preamble frames.
M*0.02/(P*0.02+M*0.02)xe2x80x83xe2x80x83(1)
In other words, if, as an example, there were 5 message frames and it took more than 2 preamble frames but less than 3 preamble frames to assure synchronization at any site within the cell defined by a given BS, the maximum efficiency would be (5*0.02)/(3*0.02+5*0.02) or 62.5%.
If the worst case time to assure synchronization with respect to a given BS for the example outlined above happens to be 2.1 frames, then {fraction (9/10)}ths of the preamble time frame is xe2x80x9cwastedxe2x80x9d each access period.
Thus far, this discussion has identified inefficiencies due to the preamble frame duration. In addition to the preamble frame duration, the frame time duration of the message capsule will also impact the overall protocol efficiency. However, there are additional constraints. For example, the message capsule frames also contain overhead bits, where usually the number of overhead bits remains constant independent of the frame duration. Thus, a decrease in the frame time for the message capsule, without proper adjustments (e.g., like increasing the message capsule data rate), could cause the overall protocol efficiency to decrease.
It would thus be desirable to provide a method whereby the time required to obtain preamble synchronization (i.e., a multiple of the preamble frame duration) is not required to be identical to the message capsule frame duration and in the most general case not required to be a frame time related multiple or sub-multiple of the message capsule frame time.
The present invention comprises the method of increasing the efficiency of data transmission, between a mobile subscriber or station (MS) and a base station (BS), by altering the frame size (time slot length) of at least one of the access channel preamble and the access channel message capsule from that standardized in the prior art.